Derivatives and Risk Mangement (Balance Sheet Classification) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Total derivative assets	$ 834,329	$ 717,343
Total derivative liabilities	14,714	12,513
Designated as Hedging Instrument [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	829,514	713,640
Derivative Liability Designated as Hedging Instrument, Fair Value	14,369	12,058
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	437,974	333,479
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Other Assets
Derivative Asset Designated as Hedging Instrument, Fair Value	211,439	201,081
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	14,341	11,849
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	180,101	179,080
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	28	209
Not Designated as Hedging Instrument [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	4,815	3,703
Derivative Liability Not Designated as Hedging Instrument, Fair Value	345	455
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Asset [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	2,636	1,906
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Assets
Derivative Liability Not Designated as Hedging Instrument, Fair Value	2,179	1,797
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Liability [Member]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	209	400
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Long-Term Hedging Liability [Member]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	$ 136	$ 55